In recent years, there has been widespreaded adaptation of highly efficient digital transmission in wireless communications such as Wideband Code Division Multiple Access (W-CDMA), Worldwide Interoperability for Microwave Access (WiMAX), Long Term Evolution (LTE) and etc. In such modern communication systems, the transmitted signals have become to be wideband and therefore these signals have a high peak-to-average power ratio (PAPR). The PAPR is defined as a ratio of a peak of a communication signal to an average of the communication signal.
In general, linear power amplifiers have more effective power efficiency as the PAPR decreases. Power amplifiers that can have a high power amplitude but have an average level far below the high power amplitude may consume higher power than that would be needed to output a predetermined average power level with a lower PAPR.
Usually, the PAPR of a signal may be reduced by clipping of the peak of the signal at a predefined level prior to pre-transmission amplification of signal with the high power amplifier (HPA). That is, the PAPR reduction unit may be provided to reduce the PAPR of the signal by clipping of the peak amplitude. Some sort of PAPR reduction prior to the amplification of signal in the PAPR reduction unit may lead to some benefits such as reduced costs and power efficiency improvement. However, such clipping of the peak amplitude of the signal in most of the PAPR reduction unit may introduce clipping noise into the output signal from the PAPR reduction unit that would be inputted into the HPA. Hence, the clipping noise included in the input signal of the HPA would be amplified. Therefore the HPA output signal became corrupted with this clipping noise. The clipping noise at the HPA output terminal increases the spectral regrowth and causes out-of-band spectrum emission.
The pre-transmission amplification of the signal may constitute one of the major costs associated with information transfer in the modern communication system. In general, while package cost dominates for low-power amplifiers, additional amplifier cost may be exponentially increased for higher peak-power amplifiers. For this reason, it should be important to reduce peak amplitude of signals to be transmitted as low as possible in the modern communication systems. This is one of the other reasons for making clipping unavoidable.
The nonlinearity of an amplifier may make the input signal energy to be converted into nonlinear spectral energy. Therefore, it would be needed to limit the peak amplitude of the input signal into the amplifier such that it only exceeds the upper limit of the linear region of amplification. After the amplifier has been linearized to a practical limit, generation of undesirable nonlinear spectral components may be reduced by clipping the peak amplitude of the input signal into the amplifier.
The need for peak reduction processing would be emphasized in so-called multi-channel signal (MCS) waveforms for wireless communication systems. Further, several important world-wide wireless standards (e.g., 802.11 (WiFi) and 802.16 (WiMAX)) have adopted orthogonal frequency-division multiplexing (OFDM) waveforms that use parallel transmission of many narrowband components. An OFDM signal has no spectral spacing between adjacent channels, and has short burst transmission rather than continuous one. The WiMAX waveform uses basestation transmissions including several hundred channels of OFDM waveforms to be allocated to many users. The large power peak variation of the many OFM channels may lead to a necessity of peak reduction processing to satisfy constraints, for example, for error vector magnitude for each of channels allocated for each user.
There are some known techniques to reduce peak amplitude of communication signal. One of these techniques is a clip-and-filter approach, in which communication signal passes through a “clipper” (i.e., hard-limiter), and then filter clipped communication signal to ensure satisfy regulatory spectral constraints. This approach is widespreadly used for peak-reduction of OFDM signals. The clipping process cause the clipped communication signal to differ its ideal shape, while infusing in-band and out-of-band noise into the clipped communication signal. The out-of-band noise generated during the clipping process would cause spectral regrowth in excess of the amounts permitted by spectral constraints. The filtering process in which the out-of-band noise in the clipped communication signal is filtered would establish the spectral constraints. However, in general, the clipping thresholds are set at relatively high values to meet a constraint for error vector magnitude, for example so that the clip-and-filter approach become not so effective. And, clipping process in the clip-and-filter approach introduces nonlinear interference, which cannot be removed by the out-of-band filtering.
Further, the relationship between peak amplitudes of signals and amplifier characteristics (it will also be referred to as amplifier curve) may be one of important characteristics of the high power amplifiers (HPAs) in the communication systems. In general, successful power amplifier may exhibit a nonlinear relationship between input power and output power. For low levels of input power, the output power of the amplifier depends on the input power linearly. However, for higher levels of input power, the output power of the amplifier may saturate at an upper limit which cannot be exceeded. The nonlinear relationship between input power and output power may appear near the saturation point at which the output power of the amplifier saturates. Operation of the amplifier near the saturation point (i.e., operation in nonlinear mode) may result in generating nonlinear distortion which can corrupt the shape of the spectral curve of the input signal. In order to reduce level of the non-linear distortion in HPA, a pre-distortion technique has been proposed to decrease the effects of nonlinearity.
In mobile communication systems, the nonlinear relationship between input power and output power may cause the frequency spectrum vicinity of the transmission frequency to have lifted side lobs so that leakage into adjacent frequency channels and adjacent channel interference may occur. The leakage power is explained as the Adjacent Channel Leakage Ratio (ACLR) or the Adjacent Channel Power Ratio (ACPR). The ACLR is normally defined as a ratio of the power of a channel to the adjacent leakage power. This leakage power is noise with respect to other channels.
Memory effects may be other phenomena related to power amplifier in the modern communication system. As the term “memory effects” means that there is dependence not only on the present sample but also on previous samples of the signal. Memory effects may be exhibited as a non-symmetrical spectrum around the central spectrum of the output signal of a power amplifier. In other words, although the desired spectrum of the signal is perfectly symmetrical, the spurious spectrum due to the distortion may be non-symmetrical with respect to the center of the signal spectrum.
Generally, HPA is operating in the nonlinear mode, and therefore, increases the level out-of-band spectral component in the output signal. The predistorter (PD) may be suppressing these nonlinear distortions due to the clipping noise, i.e., out-of-band components in order to maintain the level out-of-band spectrum at the HPA output appropriate. However the residual out-of-band spectrum due to PAPR reduction cannot be suppressed with the conventional PD because the signal with PAPR reduction (i.e. signal with high level of the out-of-band spectrum) acts as the reference signal for PD. that is, the signal corrupted by clipping noise becomes the reference signal for predistorter (PD). Thus the residual level of out-of-band components is presenting at the HPA every time despite the specific PD architecture, and even when the PD can pre-distort or linearize the reference signal to handle the non-linearity, the high level of the out-of-band spectrum, caused by the clipping noise, will translated directly to the HPA output.
The high level of the out-of-band spectrum causes interferences between the adjacent bands in the communication system. Normally the level of the out-of-band spectrum is characterized by the adjacent channel leakage ratio (ACLR) parameter. Therefore the levels of ACLR and the out-of-band emission must to be as low as possible.
Therefore, there is a need for a signal processing system and a method for handling both of the clipping noise and the nonlinear signal distortions in the HPA output signal. Further it is preferable that the memory effects caused in the HPA may be handled.